Power generation from high altitude winds

ABSTRACT

There are disclosed method and apparatus for the production of electrical power from high altitude winds. A kitecraft (12) secured to a ground tether (14) supports a cylindrical drum (10) rotatable about a horizontal shaft (60). The drum comprises a plurality of wheels (24) interconnected by airfoils (26) positioned about the cylindrical surface of the drum. Wind action on the airfoils rotates the drum about the horizontal shaft and turns generators (50) to provide electrical power. The kitecraft (12) and drum (10) assembly is constructed to the maximum practical extent from tension members to reduce weight.

TECHNICAL FIELD

This application is a continuation-in-part of my copending applicationSer. No. 372,632 filed April 28, 1982 for "Apparatus for ExtractingEnergy from Winds at High Altitudes."

This invention relates to method and apparatus for extractingsubstantial electrical power from the wind. More particularly, thisinvention relates to extracting power from winds at altitudes too highfor ground support such as, for example, between 200 and 40,000 feetabove the surface of the earth.

BACKGROUND ART

The referenced copending application discloses and claims one approachto the production of electrical energy from high altitude winds.Specifically described therein is a kite-supported module whichincorporates airfoil elements in an endless path around a pair of spacedshafts. The contents of that copending application are incorporatedherein by reference.

As pointed out in the referenced copending application, many of the windpower projects suggested by the prior art are directed to extractingpower from winds at ground level. These approaches have a basicshortcoming due to the relatively low power density and extremevariability in time and location of ground level winds. Typical of theseapproaches are wind axis "propeller" wind turbines and the cross axisDarrieus "eggbeater" wind turbines in wide use today on California andother wind farms.

The Darrieus unit employs a vertical shaft with two to five curvedblades. Another cross axis approach is the "cyclogiro" which employsstraight, adjustable pitch, airfoils rotating around a vertical axis. Intheory, the cyclogiro wind turbine has the highest predicted powercoefficients of all wind turbines (See Mark's Standard Handbook forMechanical Engineers, eighth edition, pages 9--164 through 9-166).

In addition to the approach contained in the referenced copendingapplication, other proposals have been made for extracting power fromhigh level winds. These approaches are particularly attractive becausethe energy content of wind goes up as the cube of the wind velocity. Onesuch system is shown in U.S. Pat. No. 3,924,827 of Lois which disclosesa series of buoyant wings connected by tethers to an electricalgenerator. The system is so arranged that, as one wing is beingretracted, at least one other wing is being drawn away by the wind todeliver power to the generator.

Another proposal for high altitude wind power extraction is set forth ina paper entitled "Electricity Generation from Jet Stream Wind" byFletcher and Roberts appearing in the July-August 1979 issue of TheJournal of Energy of the AIAA. That paper proposes an airplane-likestructure tethered to the ground and carrying wind turbines drivingelectric generators which deliver electrical current to the groundthrough two conductors forming part of the tether. The proposal involvesflying the device at a relatively high altitude such as 12 kilometers.

The importance of the energy potential of high level winds may bederived from world wind data for 1979-80 available from The NationalOceanic and Atmospheric Administration (NOAA). For example, at San Diegoin midwinter of 1980, the average wind speed at 40,000 feet was almost70 miles per hour, as opposed to about 8 miles per hour at sea level, 15mph at 10,000 feet, 30 mph at 20,000 feet, and 45 mph at 30,000 feet.The available power increases with the cube of the wind velocity butdecreases with air density. Set forth in the table below aretheoretically available wind power figures in arbitrary relative unitsfor San Diego for midwinter 1980 and midsummer 1979. Wind velocityfigures vary widely with time and location, and at many other locationsrelative available wind power is typically higher at 30,000 feet than40,000 feet.

    ______________________________________                                               Relative Midwinter Rela- Midsummer                                     Altitude                                                                             Air      Wind      tive  Wind    Relative                              in feet                                                                              Density  Velocity  Power Velocity                                                                              Power                                 ______________________________________                                           0   1.0       8         1     8      1                                     10,000 .74      15         5     6      0.5                                   20,000 .53      30        28    10      1                                     30,000 .37      45        66    20      6                                     40,000 .25      70        167   35      20                                    ______________________________________                                    

It will be apparent from the foregoing table that winds aloft,especially at high altitudes, have significant potential for energygeneration if that potential can be successfully harnessed.

Accordingly, it is a primary object of the present invention to provideimproved method and apparatus for harnessing the potential of highaltitude winds. Another object is to harness such energy by means of anefficient and relatively simple device which is readily positionable atthe location and altitude for favorable wind conditions. Other objects,features, and advantages will be apparent from the following descriptionand appended claims.

DISCLOSURE OF INVENTION

In accordance with the present invention, there is provided adrum-shaped apparatus rotatable about a horizontal axis and suspendedfrom a suitable support structure, such as a kite. The cylindrical drumcarries a of airfoils around its periphery which are positionable togenerate drum-rotating lift in response to prevailing winds, and as suchis probably properly classified as a horizontal axis cyclogiro. Therotation of the drum drives one or more electrical generators whichsupply electrical energy to the ground. The apparatus is connected toone or more ground tethers in a manner such that it automatically alignsitself with the prevailing winds.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of apparatus in accordance wiith theinvention, illustrated at altitude.

FIG. 2 is a side, elevational, view of the apparatus of FIG. 1.

FIG. 3 is a front view of the apparatus of FIG. 1

FIG. 4 is a detail of one of the end wheels forming a part of theinvention;

FIG. 5 is an enlarged cross-section taken substantially along the line5--5 of FIG. 2

FIG. 6 is an enlarged cross-section taken substantially along the line6--6 of FIG. 2.

FIG. 7 is an enlarged profile of one of the airfoils of the invention.

FIG. 8 illlustrates an airfoil mounted and connected to restrainingcables.

FIG. 9 comprises diagrams illustrating the operation of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

With particular reference to FIG. 1, there is illustrated an apparatusin accordance with this invention comprising a cylindrical drum 10suspended beneath a kitecraft 12, the whole being retained by a groundtether 14 so as to face into the wind, as indicated by the arrows W. Thekitecraft 12 comprises a wing 16 which may have a conventional airfoilcross-section such as NACA 4415 and a tail assembly comprising astabilator 18 and a pair of rudders 20 mounted at the end of a pair oftwin booms 22 extending from the wing. The cylindrical drum which issuspended below the kitecraft comprises a plurality of wheels 24 whichare spaced apart as illustrated in FIGS. 1 and 3 and interconnected by aplurality of airfoils 26 which essentially form the surface of a rightcircular cylinder.

In order to reduce the weight of the apparatus of the invention, it isconstructed to the maximum possible extent from tension members.Accordingly, the wheels 24 are fashioned in much the same manner asbicycle wheels in that each comprises a relatively stiff rim 28 carryingspokes 30 which operate only under tension and may be lightweightcables.

The drum 10 is suspended beneath the kitecraft 12 by means of a pair ofsupport carriages 32 engaging each of the wheels 24. Each pair ofsupport carriages depends, in fore to aft spaced relationship, from thewing 16. Each of these support carriages may include brackets 34 whichcarry a pair of rollers or tires 36 which engage the rim 28 of each ofthe wheels. The manner in which this is accomplished will be mostapparent from FIG. 5 which shows the rim 28 in cross-section. The rim isstreamlined by means of a convex outer periphery 38 in order to minimizeturbulence in advance of the airfoils. The inner surface of the rimdefines a pair of grooves 40 which receive the tires 36.

Each of the wheels 24 making up the drum also engages a generatorcarriage 42. The generator carriages are similar in some respects to thesupport carriages 32 but are located on the ground tether, or windward,side of the drum 10 possibly, for example, 105 degrees clockwise fromthe top as viewed in FIG. 2. Each generator carriage includes a supportmember 44 with four tires 46, two rolling within each groove 40 on theinnerside of the rim 28 of each wheel 24. As is seen most clearly inFIG. 6, the carriage also includes an idler tire 48 rolling upon theouter periphery of the rim 28. Idler 48 may be spring loaded to retainthe tires 46 within the grooves. A generator 50 on each generatorcarriage 42 is driven by the tires 46 through pulleys 52 and V belts 54.

The generator carriages 42 are secured in place by cable harnesses 56(FIG. 1) having tie points 57 at the kitecraft wing 16 and tie point 58at the ground tether 14. As seen in FIGS. 1 and 2, cables run from thegenerator carriages 42 to the kitecraft 12 and the ground tether 14.Cables from the kitecraft tie points 57 to the ground tether tie point58 enable the kitecraft to maintain a reasonable tether angle in spiteof the relatively high apparatus drag when power is being generated.

A shaft 60 (FIG. 4) extends through the drum 10 and beyond the lastwheel 24 on each end. The shaft carries a bearing 62 at each end. Theinner race 64 of the bearing 62 is secured to the shaft while the outerrace 66 is connected to the forward and aft wing tip by cables 68, andto the tether 14 by cables 70 (FIG. 3). The end of the shaft 60 adjacentthe inner race 64 is also connected to the nearest wheel rim 28 by meansof radial cables 72. These cables 72 maintain the length dimension ofthe drum 10 and skew and torsion of the drum are avoided by means ofcrisscross cables 74 (FIG. 3) between the wheels 24. Some of thesecrisscross cables are attached to the shaft, serving as well to keep itfrom buckling. In order to reduce drag, these cables may be made of avery thin but strong material such as, for example, Kevlar.

In order to simplify the drawing, the drum 10 is illustrated as havingtwelve airfoils 26. However, a greater or lesser number may also beemployed. In an illustrative embodiment, each of these foils 26 has anNACA 0015 cross-section with a chord of approximately four feet. Exceptfor slits at the spokes, each foil extends completely through the drum10 but is mounted on, and rotatable about, aligned spokes 30 of the drumwheels 24 near the rims 28.

A typical foil profile is illustrated in FIG. 7. The foil 26 is designedwith its heaviest structural members at the foil's leading edge suchthat the center of gravity (CG) is located forward of its aerodynamiccenter (AC). The foil 26 is connected to spokes 30 by pivots 76 (FIG. 8)at the foil's cross sectional center of gravity. A stiffening cable 78extends from each pivot 76 to an adjacent spoke 30 to assist intransmitting rotational force to the rim 28. The aerodynamic center (AC)may be defined as the point about which the moment of the air forcesremains constant when the angle of attack is changed. Since each foil ismounted to the spokes at its center of gravity, in the absence of wind,it will remain in any preestablished position. In the presence of wind,however, the foil will align itself with its aerodynamic center directlydownwind from its center of gravity. This is an important featurebecause the wind itself may be used to orient the foils 26 into theproper attitudes for power generation. It is also important that thefoils be mounted exactly at their centers of gravity in order thatcentrifugal forces do not affect their rotational positions.

In order for a foil to generate "lift," and therefore power, the foilmust have a positive angle of attack relative to the apparent wind.There are various ways of obtaining this positive angle of attackincluding, for example, computerized motor control. However, one simpleand effective method employs restraining cables 80a, b as illustrated inFIG. 8. Each cable incorporates a respective tension spring 82 and isconnected between the trailing end of a foil 26 and an adjacent spoke30. Cable 80a is connected to limit clockwise rotation of the foil asviewed in FIG. 8. It controls the attack angle of the foil on thewindward side of the drum 10. Cable 80b limits counterclockwise rotationand controls attack angles on the leeward side of the drum 10. Thespring force, cable lengths and cable attachment points to the spokesare selected to make the restraining force result in efficient angles ofattack for the conditions under which the apparatus is to be operated.When a foil is thereby restrained and prevented from aligning itselfdirectly into the apparent wind, a lift force will be generated from theresulting angle of attack. This lift force acts perpendicular to thedirection of the apparent wind. As is true of most wind turbines, thegreatest efficiency is achieved when the foils are travelling at alinear speed faster than that of the wind velocity. The component of thelift force perpendicular to the rim is thus larger than that along therim. The force parallel to the rim would be unbalanced but, in theapparatus of this invention, as in other wind turbines, this force iscounteracted by a generator. Maximum efficiency in the describedembodiment is expected to be achieved with a rim speed of approximatelytwice wind speed.

In order to simplify the drawing, all the foils 26 shown in FIG. 2 arealigned parallel with rim 28. In actuality, good results may be achievedby holding the angle to within approximately 10 degrees of parallel.This may be best understood by referring to the vector diagrams of FIG.9. These diagrams assume the apparatus of the invention to be flying ina 30 mile per hour wind the drum rotating at a speed of 60 miles perhour at the foils. It is also assumed that optimum lift for the airfoilin use is an angle of attack of 20 degrees when operating in thepresence of other airfoils at the spacing shown. The diagrams illustratethe conditions existing at 3 points about the periphery of the drum 10as viewed in FIG. 2. One diagram (a) illustrates conditions at the 90degree position or directly to windward. A second (b) illustratesconditions at 270 degrees or directly to leeward and a third (c)illustrates conditions at 150 degrees, a position 30 degrees windward ofbottom center. In each instance, the airfoil is shown with an angle ofattack of 20 degrees into the apparent wind. The lift vector is notdrawn to scale but illustrates the direction of lift, which isperpendicular to the apparent wind. In each instance, it will be notedthat the direction of the lift vector is such as to include a componentparallel to the rim in a direction to induce counterclockwise rotationabout the central shaft. (In FIG. 9, the lift vector is elongated tomore clearly illustrate its components.) At both the 90 degree and 270degree positions, the angle of the airfoil is 6.6 degrees from a tangentto the rim of the wheel 24. At 150 degrees, it is within 0.1 degree ofbeing tangential.

It will be understood by those skilled in the art, that the principlesof this invention are applicable to devices of many sizes and powergenerating capabilities. However, a specific example may be considered.For example, if a drum is designed to have peak efficiency in a 30 mileper hour wind, it is believed that the optimum foil speed should beapproximately 60 miles per hour or 88 feet per second. Considering adrum with the foils at a diameter of 50 feet, it can be readilycalculated that the resulting centrifugal force will amount to 9.6 g.Assuming an airfoil 26 having a chord of 4 feet and weighing 2 poundsper lineal foot, the centrifugal force required to retain it will be19.2 pounds per lineal foot of span. A spacing of 25 feet between thewheels 24 should provide little interference to the wind on the foilswithout requiring tremendous foil strength due to length of unsupportedspan.

Employing 12 airfoils as in the illustrated embodiment, it is estimatedthat the horizontal drag force at the generator carriages 42 wouldamount to approximately 3,400 pounds for each 25 foot drum section whenpower is being generated. Making allowance for the weight of the wheelrims 28, the foils, shaft, and cabling, the total weight per drumsection would be approximately 1,100 pounds. Using these figures, it canbe calculated that each of the generator carriage tires 46 would have aload of about 850 pounds. For an apparatus of this size, the wing 16 ofthe kitecraft could have a span of approximately 300 feet and a chord of60 feet.

There are at least five situations in which it is undesirable to havethe drum 10 rotating. These include:

1. during ascent;

2. during periods of high turbulence;

3. during periods of low wind when it is desired maintain altitude.

4. during descent, as when being towed in by a winch to prevent loss oftether tension during periods of very low wind; and

5. during potential or actual problems of malfunction aloft.

In all of these situations, the most suitable airfoil positions would beeither at an angle of attack which will provide good lift with minimumdrag or an angle that will give minimum drag and minimum lift.

If the drum 10 is rotating, the minimum drag, minimum lift status, maybe achieved by releasing the tension on the restraining cables 80. Thismay be done, for example, by radio control. The wind will then cause thefoils to assume their free floating position parallel to the wind andthe drum 10 will stop rotating. The foils may then be locked in thisposition by radio control and the drum rotated, for example to 10degrees, by the generators acting as motors to thereby position all theairfoils 26 into positions of vertical lift which would be additive tothe lift of the overhead wing.

To activate drum rotation while aloft, the foils may be unlocked byradio control and rotation of the drum 10 started using the generatorsas motors. Each foil at some point in the cycle will become alignedparallel with the rims 28 and at this point tension is applied to therestraining cables 80. This will produce drum rotation and powergeneration.

The apparatus of this invention may be flown to altitude in the samemanner as a kite. However, for jet stream power generation, if there isvery little wind at lower altitudes, a propulsion device, eitherintegral with, or separate from, the apparatus may be used to advantageto raise the apparatus to a height capable of maintaining tetheredflight. By stopping rotation of the drum and setting the airfoils 26 forlift, as explained above, the drag due to power generation iseliminated, and with a device of the size described above, some 9,600square feet of foil lift is added. This would provide lift-off at sealevel at a wind speed of only about 13.5 miles per hour, but thecorresponding altitude maintenance speed at 30,000 feet is about 22 mphbecause of the much lower air density.

The apparatus described above is relatively lightweight because itsshape is maintained almost entirely by cables under tension, rather thanby compression members. Furthermore, since the generators are drivenfrom the rims 28 of the wheels, high speeds can be obtained, therebypermitting the use of light generators whose power to weight ratios varywith design operating speed. There is no need for gear boxes, a heavyitem in some conventional wind turbines.

The described apparatus should be able to generate about 600 KW in a 30mph wind at an altitude of 500 feet. Essentially the same apparatus witha rim speed of 100 mph should be able to generate one megawatt of powerin a 50 mph wind at 30,000 feet.

It is believed that the many advantages of this invention will now beapparent to those skilled in the art. It will also be apparent that anumber of variations and modifications may be made therein withoutdeparting from its spirit and scope. Accordingly, the foregoingdescription is to be construed as illustrative only, rather thanlimiting. This invention is limited only by the scope of the followingclaims.

What is claimed is:
 1. Apparatus for extracting energy from wind at high altitude, comprising:means for supporting a load at said high altitude; means for flexibly tethering said load supporting means to the surface of the earth; a plurality of elongated parallel airfoil elements supported by said load supporting means, said airfoil elements lying on the surface of an imaginary horizontal circular cylinder and rotatable about the horizontal axis of said cylinder in a closed cylindrical path; means for positioning said airfoil elements relative to said cylindrical surface whereby relative wind substantially normal to said axis exerts a force on said airfoil elements, causing said elements to rotate about said horizontal axis; and means responsive to the movement of said airfoil elements about said horizontal axis for transmitting power to the surface of the earth.
 2. The apparatus of claim 1 wherein the airfoil element positioning means is adapted to alternatively position said elements in substantially fixed location about the surface of said cylinder with the aerodynamic lift of the airfoil elements adding to the support provided by the load supporting means.
 3. The apparatus of claim 1 wherein said airfoil elements are secured to a plurality of wheels of substantially equal diameter suspended from said load supporting means, said wheels lying in substantially parallel planes spaced along the horizontal axis of said cylinder and rotatable about said horizontal axis.
 4. The apparatus of claim 3 wherein:said positioning means pivotally secure the airfoil elements to the wheels whereby each airfoil element is rotatable about an attack-angle axis passing substantially through its center of gravity and substantially parallel to said horizontal axis.
 5. The apparatus of claim 4 wherein said airfoil elements are substantially identical and wherein each said element has an aerodynamic center displaced downwind from its center of gravity.
 6. The apparatus of claim 5 including means for pivotally positioning each of said airfoil elements about its center of gravity.
 7. The apparatus of claim 6 wherein said pivotal positioning means comprises means for limiting the rotation of each said airfoil element.
 8. The apparatus of claim 3 additionally comprising:at least one of said wheels having a circular rim defining a groove encircling its inner surface, and a smaller support wheel engaging and rollable in said groove with its axis secured to said load supporting means for suspending said wheel therefrom.
 9. The apparatus of claim 8 wherein said rim defines a pair of axially spaced, coaxial grooves, and wherein at least one support wheel engages each of said grooves.
 10. The apparatus of claim 3 wherein each of said wheels comprises:a hub; a substantially rigid rim; and a plurality of tension spokes interconnecting said hub and said rim.
 11. The apparatus of claim 3 wherein at least one of said wheels has a circular rim defining a groove encircling its inner surface and said movement responsive means comprises:a smaller drive wheel engaging and rollable in said groove; an electrical generator; and means for transmitting power from said drive wheel to said generator.
 12. The method of developing power from the wind at relatively high altitudes comprising:raising a load supporting assembly to a high altitude; tethering said load supporting assembly in a substantially fixed position relative to the earth; supporting with said assembly a plurality of elongated parallel airfoil elements lying on the surface of an imaginary horizontal circular cylinder and movable about the axis of rotation of said cylinder; moving said elements about said axis of rotation by wind action; and generating electrical power from the movement of said elements. 